1. Field of the Invention
The present invention relates to an electron beam exposure system where an electron beam is irradiated onto a target through a mask having a plurality of repetitive pattern openings, and the electron beam having the plurality of repetitive patterns after having passed through the mask is irradiated onto the target.
2. Description of the Related Art
In a first prior art electron beam exposure system, a variable-shaped beam has been used. That is, a first mask having a first rectangular opening, a second mask having a second rectangular opening, and deflection units arranged between the first and second masks are provided. An electron beam passes through the first rectangular opening of the first mask and the second rectangular opening of the second mask, to form a rectangular cross-section of the electron beam. This rectangular cross-section of the electron beam is varied by the deflection units, to obtain a variable rectangular cross-section of the electron beam. This will be explained later in detail.
In the first prior art electron beam exposure system using a variable shaped electron beam, however, when exposing a plurality of the same patterns such as cells of a dynamic random access memory (DRAM) device, a large number of deflecting operations are required, which reduces the throughput of the system.
In order to enhance the throughput of the system, a second prior art electron beam exposure system using an electron beam having a bundle of patterns is known (see FIG. 3 of JP-A-HEI 3-174716). That is, an electron beam is irradiated onto a mask having a plurality of patterns, and the electron beam having that plurality of patterns after having passed through this mask is irradiated onto a target. Thus, the plurality of patterns are simultaneously exposed on the target to remarkably enhance the throughput of the system. Also, in the second prior art electron beam exposure system, shields such as fine meshes, each smaller than a resolution limit value, are provided within each pattern opening of the mask, to substantially reduce the current density of the electron beam. This will be explained later in detail. Thus, a proximity effect among the patterns due to the difference in energy deposited within a resist is corrected.
This proximity effect is explained next. Generally, the deposited energy E(r) of the electron beam within the resist is represented by the following double Gaussian expression: ##EQU1##
where r is a distance between a calculated point and an irradiation point of the electron beam;
.beta..sub.f is a forward-scattering range due to the electron scattering within the resist;
.beta..sub.b is a backscattering range due to the electron scattering from the boundary between the resist and its underlying substrate; and
.eta. is a ratio of the energy deposited by backscattering to the energy deposited by forward-scattering.
Even in the above-described second prior art beam exposure system, when the target has a plurality of the same patterns whose number is not a multiple of the number of the patterns of the mask, the first prior art electron beam system using a variable-rectangular electron beam has to be used for peripheral portions of the target, thus reducing the throughput of the system.
Also, the shields provided within each of the pattern openings of the mask for correcting the proximity effect are very difficult to manufacture. Also, the shields lower the mechanical strength of the mask.